Organic electroluminescence (EL) devices are divided into two types, i.e. a fluorescent type and a phosphorescent type. For each type, an optical device design has been studied according to the emission mechanism. For the phosphorescent organic EL device, it is known that due to its emission properties, a high-performance device cannot be obtained by simple application of the fluorescent device technique. The reason therefor is generally considered as follows.
The phosphorescent emission utilizes triplet excitons and thus a compound used in an emitting layer has to have a large energy gap, since the energy gap value (hereinafter also referred to as singlet energy) of a compound is normally larger than the triplet energy value (referred to as the difference in energy between the lowest excited triplet state and the ground state in the invention) of the compound.
Therefore, in order to confine the triplet energy of a phosphorescent dopant material in an emitting layer efficiently, it is required that a host material having a larger triplet energy than that of a phosphorescent dopant material be used in the emitting layer. In addition, it is required that an electron-transporting layer and a hole-transporting layer be provided adjacent to the emitting layer, and compounds having a triplet energy larger than that of the phosphorescent dopant material be used in the electron-transporting layer and the hole-transporting layer.
As seen above, designing an organic EL device based on the traditional design concept leads to the use in the phosphorescent organic EL device a compound having a larger energy gap than that of a compound used in the fluorescent organic EL device, thereby to increase the driving voltage of the whole organic EL device.
In addition, a hydrocarbon-based compound having a high oxidation resistance and a high reduction resistance, which is useful for a fluorescent device, has a broad pi-electron cloud, and hence a small energy gap. Thus, for the phosphorescent organic EL device, such a hydrocarbon-based compound is unlikely to be selected, but an organic compound containing a hetero atom such as oxygen or nitrogen is rather selected. Consequently, the phosphorescent organic EL device has a problem that it has a shorter life as compared with the fluorescent organic EL device.
Further, the device performance is greatly affected by the fact that the relaxation rate of triplet excitons of a phosphorescent dopant material is very slower than that of singlent excitons thereof. That is, the emission from singlet excitons is expected to be efficient, since the rate of the relaxation leading to the emission is so rapid that excitons are unlikely to diffuse to the neighboring layers of an emitting layer (hole-transporting layer or electron-transporting layer, for example). On the other hand, since emission from triplet excitons is spin-forbidden and has a slow relaxation rate, the triplet excitons are likely to diffuse to the neighboring layers, so that the triplet excitons are thermally energy-deactivated unless the phosphorescent dopant material is a specific phosphorescent compound. In short, in the phosphorescent organic EL device, control of electrons and holes in the recombination region is more important as compared with the fluorescent organic EL device.
For the above reasons, enhancement of the performance of a phosphorescent organic EL device requires material selection and device design different from those of a fluorescent organic EL device.
In addition, when a device has a structure in which the pi-conjugated bond is cut in order to enhance the triplet energy of a compound, the transporting property of carriers tends to lower. That is, the pi-conjugated bond is required to be extended for higher transporting property of carriers. However, doing so causes a problem that the triplet energy is lowered.
Under such conditions, Non-Patent Document 1 discloses the use of an N-fluorenyl carazole compound as a material for an organic EL device, for example.